How to force a factory reset or hard reset on Oppo devices

Oppo Find 7a Oppo Find 7a
Oppo Find 7 Oppo Find 7
Oppo R3 Oppo R3
Oppo N1 mini Oppo N1 mini
Oppo Neo 5 Oppo Neo 5
Oppo R1001 Joy Oppo R1001 Joy
Oppo R2001 Yoyo Oppo R2001 Yoyo
Oppo Find 5 Mini Oppo Find 5 Mini
Oppo Neo 3 Oppo Neo 3
Oppo R1S Oppo R1S
Oppo N3 Oppo N3
Oppo R5 Oppo R5
Oppo U3 Oppo U3
Oppo R1x Oppo R1x
Oppo A31 Oppo A31
Oppo Mirror 3 Oppo Mirror 3
Oppo Joy Plus Oppo Joy Plus
Oppo Neo 5s Oppo Neo 5s
Oppo Neo 5 (2015) Oppo Neo 5 (2015)
Oppo R7 Oppo R7